1. Technical Field
The invention relates to a process for recovering hydrocarbons from a subterranean formation, and more particularly to a hydrocarbon recovery process using a permeability-reducing agent.
2. Background Information
The utility of crosslinked polymer gels as a permeability-reducing agent in subterranean hydrocarbon-bearing formations to facilitate hydrocarbon recovery therefrom has long been known in the art. U.S. Pat. No. 3,762,476 to Gall is representative of conventional teaching in the art of its time. Gall discloses a conformance improvement treatment process, wherein a permeability-reducing crosslinked polymer gel is formed in situ by injecting an aqueous slug containing a crosslinkable water-soluble polymer into the subterranean treatment region followed in sequence by an aqueous slug containing a crosslinking agent made up of a polyvalent metal cation complexed with a retarding anion. Useful polyvalent metal cations listed in Gall include iron(II), iron(III), aluminum(III), chromium(III), calcium(II), and magnesium(II). Useful retarding anions complexed with the cation include citrate, phosphate, acetate, nitrilotriacetate, and tartrate.
Sequential injection of the gel components as separate and distinct slugs into the treatment region is essential to the teaching of treatment processes such as Gall insofar as surface contacting of the polymer and crosslinking agent was believed to cause premature gelation of the gel components prior to reaching the treatment region. If prematurely formed at the surface or in the well bore, placement in the treatment region of the gels taught by Gall was difficult, if not impossible, to achieve. Consequently, sequential injection of the gel components in concept avoided premature gelation by delaying contacting of the components until they were displaced out into the treatment region of the formation.
In situ contacting of the polymer and crosslinking agent as required by Gall, nevertheless, proved operationally unattractive in many hydrocarbon recovery applications because of the difficulty in achieving adequate mixing of the gel components in situ. Without adequate mixing, the gels of Gall were poorly formed, resulting in weak and unstable gels that performed ineffectively as permeability-reducing agents.
In the face of conventional teaching represented by Gall, U.S. Pat. No. 4,683,949 to Sydansk et al. identified specific gel components and gelation parameters for a crosslinked polymer gel having utility in a conformance improvement treatment process, whereby the polymer and crosslinking agent of the gel could be mixed at the surface in a homogeneous gelation solution and placed in the treatment region by injection therein as a single slug. Thus, Sydansk et al. overcame the inherent operational limitations of processes such as Gall that required sequential injection and in situ mixing of the gel components. Sydansk et al. was predicated on the finding that a chromium(III)/carboxylate complex crosslinking agent could be mixed with a crosslinkable polymer at the surface to form a gelation solution producing gels that were uniquely stable and highly predictable, yet capable of practical long-term placement in subterranean treatment regions, even in hostile environments. The process of Sydansk et al. successfully enabled formulation of gels having a broad range of physical characteristics for effective conformance improvement treatments under varying in situ conditions.
The teaching of Sydansk et al. has subsequently been adapted to certain specific, typically problematic, conformance improvement treatment applications. For example, in some high-temperature or far well bore applications, it has been found advantageous to include a supplemental delaying agent in the gelation solution along with the chromium(III)/carboxylate complex taught by Sydansk et al. The delaying agent avoids excessive crosslinking of the polymer before the gelation solution reaches the desired treatment region, which would otherwise inordinately increase the difficulty of placing the gel in the treatment region.
U.S. Pat. No. 4,706,754 to Smith discloses the addition of a specific supplemental delaying agent to a gelation solution containing a water-soluble crosslinkable polymer and a metal/carboxylate complex crosslinking agent. The supplemental delaying agent comprises excess carboxylate anions in the form of a carboxylic acid which delay gelation of the polymer and crosslinking agent, thereby facilitating heretofore problematic placement of the crosslinked gels in high-temperature or remote treatment regions.
U.S. Pat. Nos. 5,131,469; 5,143,958; 5,219,475 and 5,219,476 to Lockhart et al. likewise teach gelation solutions containing a crosslinkable polymer, a crosslinking agent, and a supplemental delaying agent. The delaying agent is selected from one of several specified ligands in the form of an organic acid. In distinction to Smith, however, the Lockhart et al. patents teach a crosslinking agent having the crosslinking metal cation initially in the form of a water-soluble inorganic chromium(III) salt, rather than an organic metal/carboxylate complex as in Smith.
It is nevertheless apparent that the Smith and Lockhart et al. patents all require the addition of a discrete delaying agent to a conventional gelation solution, such as taught by Sydansk et al., to satisfactorily retard the gelation rate of the gelation solution and enable placement of the resulting gel in a wide range of subterranean treatment regions. Lockhart et al., "A New Gelation Technology For In-Depth Placement Of Cr.sup.3+ /Polymer Gels In High-Temperature Reservoirs," SPE/DOE 24194, teaches the formation of crosslinked polymer gels, wherein the crosslinking agent is a complex containing malonate and chromium(III) in a molar ratio of 3:1. This complex produces moderate gelation delays. However, if extended gelation delays are desired, it is necessary to supplement the crosslinking complex with the addition of a separate delaying agent to the gelation solution. The supplemental delaying agent taught by Lockhart et al. is uncomplexed malonate.
It has been found in accordance with the present invention, however, that similar gelation delays can be achieved for crosslinked polymer gels without the addition of a discrete delaying agent to the gelation solution. Accordingly, the present invention addresses a need that has existed for an effective gelation solution that can be formulated in the absence of a delaying agent, yet has a sufficiently retarded gelation rate for effective placement in high-temperature treatment regions.
It is, therefore, an object of the present invention to provide a process of forming a crosslinked polymer gel for permeability reduction in a subterranean hydrocarbon-bearing formation. It is also an object of the present invention to provide a gelation solution composition that forms such a crosslinked polymer gel. It is a further object of the present invention to retard the gelation rate of the gelation solution by employing a precursor complex in the gelation solution that is essentially inactive with respect to the polymer at ambient surface temperatures, yet transforms to an active crosslinking agent complex with respect to the polymer at elevated temperatures experienced in certain subterranean hydrocarbon-bearing formations. It is another object of the present invention to control the degree of gelation rate retardation by adjusting the relative proportions of chemical species within the precursor complex of the gelation solution.